After being cocultured with exosomes from resistant cells, chemo‐sensitive MCF‐7 cells exhibited decreased sensitivity to cytotoxic agents, along with increased intracellular miR‐222 and reduced PTEN level.31 Our results, together with previous lab findings, might provide hints to the role of miR‐222 in chemo‐resistance of breast cancer. This evidence concerns the gene PTEN and breast cancer.